Open pit coal mining at Phulbari has been advocated and championed by vested quarters from the very beginning. The argument for open pit hinges upon coal recovery rate. But there are a number of other more serious technical
issues that must be considered,
writes Dr Md Rafiqul Islam

There are two principal mining methods applied in the coalmines worldwide — underground (longwall, room-and-pillar) mining method, and surface or, open pit method. The longwall method has several systems, currently in operation in many countries — single-slice, multi-slice, top coal caving (goafing), hydraulic fill and pneumatic fill. All these methods in longwall systems are currently applied in Turkey, Australia, China, India, Poland, South Africa and other coal producing countries. Although open pit mining is frequent in Australia and Germany, and some places of the USA, Canada and South Africa, it has created multi-dimensional waste impacts on their surrounding environments.
   Currently coal is the second major alternative energy resource in Bangladesh. There are approximately 13 coal basins in the country that contains over 3,000 million tonnes of high-volatile B bituminous rank coal. Out of the 13, only the Barapukuria coalmine is in production by multi-slice longwall mining. Recently, some unprofessional people, lacking knowledge on geology, hydrogeology, and mining depths of ore body, are shouting for open pit mining in the Phulbari, Barapukuria, and other coal basins in northwest Bangladesh. For a while, they used examples of open pit coalmines of Germany and Australia. Their viewpoints was based only on the potential recovery rate (about 90 per cent) of the open pit mine. The author doesn’t appreciate why some people are interested in only high recovery rate. Why do they not think about the depth of the coal seam, destruction of regional mega-aquifer system, environmental degradation, replacement of densely populated towns and many villages, and forever waste of valuable arable land of the country?
   The author focuses here on the geology and hydrogeology of the Bangladeshi coal basin compared to that of the Hambach open pit mine of Germany. He also highlights the depths of open pit mine in Australia and India. Geologically, the rocks strata sequence of the coal basins in northwest Bangladesh is divided into four formations based on the age and lithology- Madhupur/Burind clay from the recent Holocene era; a water-bearing Dupi Tila aquifer of Late Miocene-Middle Pliocene age; the Permian coal-bearing Gondwana rock sequences; and Pre-Cambrian Archean basement. The Dupi Tila aquifer is further divided into two formations- the Upper and the Lower. The Upper Dupi Tila Formation consists mainly of sandstones, siltstones, and minor clay. The Upper Dupi Tila is an unconfined mega-aquifer that covers the entire northern districts and is the main source of drinking water, irrigation, and other utilities. The thickness of this formation ranges between 95-126m in the Barapukuria basin, and up to 120m in the Phulbari basin. The Lower Dupi Tila Formation (LDTF) consists mainly of loose clay with mica, pyrite, and chalcopyrite minerals. The maximum thickness of LDTF in the Barapukuria basin is about 80m to the southern part of the basin, however, to the north it is almost absent. In the Phulbari basin, thickness of the LDTF is about 30m. On the other hand, the Hambach coal deposit of Germany holds about 85 km2 areas with lignite reserves of some 2500 million tonnes. The thickness of the coal seam is up to 70m. The rock overlying the coal seam has a thickness varying between 150m in the west and 400m in the east. The basin has only a few small faults, with vertical displacements seldom more than 15m. Confined aquifers are present in large extents of the basin, which makes it nearly impossible to mine the deep seams of the Hambach mine without extensive dewatering first. The aerial extent of the groundwater table for the Hambach coalmine was estimated to be about 850 km2.
   It is rational to remind here that the Hambach mine of Germany lies beneath a confined aquifer. In contrast, the Phulbari, Barapukuria, and other Gondwana coal basins in Bangladesh are beneath an unconfined mega aquifer, the Dupi Tila Formation. Therefore, geology and hydrodynamics of the Bangladeshi coal basins are completely different from that of the German coal basin. The geological structure of the Hambach basin reveals that the basin has few faults that are of less than 15m displacement, i.e. the basin is not located within tectonically high-disturbed zone. In contrast, for the Phulbari basin, BHP drilling recognised two giant faults having vertical displacement about 150m. Existence of two major faults reveals the basin is a part of the tectonically high-disturbed zone within the continental Indian plate, where convergence and subduction of Indian plate is continuing beneath the Eurasia plate at 55 mm per year. It means there are numerous faults, fractures, joints, and fissures within the Phulbari basin. For underground mining in Barapukuria, Lower Dupi Tila aquiclude acts as barrier against the vertical inflow of water into the mine. In contrast, if an open pit mine is considered in the Phulbari and Barapukuria basins, then overburden of water will flow in frequently through its horizontal and vertical directions. Since there were no barriers in open pit mine to protect the direct inflow of huge amounts (about 50,000-70,000 liters per second) of water from the Upper Dupi Tila (UDT) aquifer through its 120-126m thick exposed rock strata to the mine pit, it would cause rapid drawdown of regional water table. This is very dangerous for the regional stability of the UDTF aquifer and it may cause devastating environmental degradation due to abrupt change of regional hydrodynamics.
   The mining methods in Australia, India, Poland, Germany, and elsewhere are usually selected based on the depth of coal seams, geographic and topographic locations of the mining area. Open-pit mines elsewhere are not located within the arable and densely populated areas. The depths of open pit mines in Australia, even in our neighbouring country, India, are shallower, and range from 60m to 150m. If the mining areas are located on high topography, for example, in Australia, the depths of open pit mines should never exceed over 150m. In contrast, if the mining area is located on low topography associated with mega-aquifer system, like in northwest Bangladesh, the open cast mining depth should not exceed 70m. For example, the Jharia coal field in India, open pit mining has a maximum depth of 70m. After that depth, underground mining continues. The depth of coal-bearing rock formations of coal basins in Bangladesh varies from basin to basin. In the Barapukuria basin, the upper coal sequence (seams I to V) occurs at varying depth from 133m to 336m, while the present mineable seam VI occurs at depth from 135m to 420m. In the Phulbari coal basin, depth of the two coal seams range between 150-260m (BHP report), or 120m-350m (GDH report), approximately. In the Khalaspir, Dighipara, and Jamalgonj basins, the depths of coal seams range between 257-483m, 328-422m, and 650-1158m, respectively.
   Therefore, the first problem for open pit mining relates to the depths of coal seams. In 1997, the depth relevant problem for open pit mining in the Phulbari basin was detected by BHP, an Australain mining company. According to an official consultant geologist of BHP’s Bangladesh Coal Project (New Age, September 14, 2008), the company recognised the depths of coal seam ranges from 150m to 260m with very high water inflow within the overburden Dupi Tila aquifer. BHP’s target was to have coal seams under a depth of maximum 100m below the surface. However, the company could not locate a shallow coal deposit around 100m depth. After the pre-feasibility study, BHP abandoned the open pit mine project in the Phulbari coal basin because BHP did not want to create another environmental disaster as they have bitter experience in Papua New Guinea. BHP is a well-experienced mining company and has several open cast mining projects in Australia. If BHP did not find scientific and logical points for open pit mining in Phulbari, being an inexperienced mining company, how does Asia Energy Corporation wish to implement their open pit mining plan in the same area? Asia Energy has no previous mining experience in Bangladesh, or anywhere else for that matter. Perhaps the company was established particularly for open pit mining in Phulbari.
   The second problem for open pit mining is related to the numerous intra-basinal and boundary faults of the Gondwana coal basins in Bangladesh. The author has experiences with the 5km long Eastern Boundary Fault with numerous intra-basinal faults, joints and fissures in the Barapukuria basin. The coal basins in northwest Bangladesh are truncated by numerous boundary faults and intra-basinal faults, and these faults are directly or indirectly connected with overburden mega-aquifer, the Upper Dupi Tila Formation (UDPF). All of the northwest coal basins developed under rift tectonic environment of subducting Indian plate beneath the Eurasia plate. Moreover, the basins were affected by highly altered igneous intrusions. Eastern Boundary Fault of the Barapukuria basin reported by Wardell Armstrong, and a Southern Boundary Fault with a high-angle central fault of the Phulbari basin, as found in GHD report (2004), are well known to the geology and mining professionals of Bangladesh. Boundary faults are very dangerous and should always be avoided for both the underground and open pit mining plan. If the boundary faults were activated due to regional earthquakes during the mine-life time, it would cause tremendous water inrush associated with high drawdown of the regional water table. It should be mentioned that the coal basins of the country are located within a highly active seismic zone. The present underground mine plan in Barapukuria excludes the Eastern Boundary Fault. However, for the proposed opencast mining in the Phulbari basin, the Southern Boundary Fault associated with the central fault is most likely included in the mine plan. This would be a dangerous problem. Because boundary faults are very long with high throw and almost vertical. These faults are connected with a network of numerous joints, fissures, and fractures of coal basins. Major faults are connected with the surface drainage systems like rivers. The Choto Jamuna river passes through the Phulbari area and it is assumed that the river course is connected with the two major faults of the Phulbari coal basin.
   The third problem is related to the mineral contents in coal as well as Lower Dupi Tila rocks. Mining professionals know well that the overburden of rocks of Upper Dupi Tila, clay-bearing Lower Dupi Tila are associated with some coal parting should be dumped during open pit mining. This author observed numerous pyrites and chalcopyrite inside the coals of the Barapukuria mine. These minerals are the source of acid mine drainage. The Lower Dupi Tila aquiclude is a clay mineral enriched rock formation and it would cause another source of AMD. Other environmental problems are excluded here because of lack of space.
   Therefore, it is obvious from the study that the overburden depths of coal seams, geology, and hydrogeology of the Gondwana coal basins don’t allow for open pit coalmines in northwest Bangladesh. Several incidents like — water inrush in 1998, gas emission in 2005, and recently surface subsidence in the Barapukuria coalmine area, are being used as negative issues against underground mining in northwest Bangladesh. For Barapukuria coalmine, where the mineable seam thickness is about 36m, multi-slice longwall mining method is applied. As a result, about 1-2m surface subsidence occurred after the extraction of first-slice of coal from the mining faces. Of course, the control of surface subsidence of the Barapukuria mining area is an important issue. In fact, the backfilling technology can play an important role to prevent land subsidence. For example, Jharia coalmine in Bihar of India manages surface subsidence by about 94 per cent by using hydraulic backfilling. Many coalmines in Poland are using hydraulic and pneumatic filling technology for deep underground mining. Therefore, why are we not taking into account their technology for our coalmines? Some questions will rise regarding economic disadvantages of backfilling technology. In this regard, it is logical to respond that if the backfilling technology is not economically viable, why do India and Poland use the technology? The coal deposit in Barapukuria currently mined is at a depth down to about 420m below surface, and backfilling can minimise surface subsidence. It can also reduce the hazard of spontaneous combustion, gas emission, water inflow into the mining tunnel. It can enhance extraction, particularly in wide coal seams that are mined in multi-slices or successive slices. Longwall top coal caving mining method associated with descending order of extraction is now being used in many underground coalmines in Poland, Turkey, China, and Australia. Consequently, if the main purpose is to use coal, the government should think about other alternatives for mining, like- longwall top coal caving method associated with backfilling technology, where the recovery rate is almost 70-80 per cent. Besides, recent clean coal technology like underground coal gasification, and coal bed methane should be considered to resolve the current power crisis of the country. The UCG and CBM technologies are environmentally favourable and used in many countries. It will not take long to implement the project and begin production. In this case, there would be no need to relocate the towns, villages and it will not destroy our valuable arable land and properties.
   The article has been written based on this author’s professional and research experiences in coal mining in Bangladesh. From a sense of sincere patriotism one feels compelled to point out the dangers of open pit mining from geological and hydrogeological points of views. The natural resources of the country, that the people own, must be used for the benefit of the people, not for the benefits of some particular quarters in and outside the country, not for some corporation, or a controversial inexperienced mining company like Asia Energy, which is more like a 21st century reincarnation of the East India Company.
   Dr Md Rafiqul Islam is a professional mine geologist